Para-xylene and meta-xylene are important raw materials in the chemical and fiber industries. Terephthalic acid derived from para-xylene is used to produce polyester fabrics and other articles which are in wide use today. Meta-xylene is a raw material for the manufacture of a number of useful products including insecticides and isophthalic acid. One or a combination of adsorptive separation, crystallization and fractional distillation have been used to obtain these xylene isomers, with adsorptive separation capturing a great majority of the market share of newly constructed plants for the dominant para-xylene isomer.
Processes for adsorptive separation are widely described in the literature. For example, a general description directed to the recovery of para-xylene was presented at page 70 of the September 1970 edition of CHEMICAL ENGINEERING PROGRESS (Vol. 66, No 9). There is a long history of available references describing useful adsorbents and desorbents, mechanical parts of a simulated moving-bed system including rotary valves for distributing liquid flows, the internals of the adsorbent chambers and control systems. The principle of using a simulated moving bed to continuously separate the components of a fluid mixture by contact with a solid adsorbent is as set forth in U.S. Pat. No. 2,985,589. U.S. Pat. No. 3,997,620 applies the principle of the simulated moving bed to the recovery of para-xylene from a feed stream containing C8 aromatics, and U.S. Pat. No. 4,326,092 teaches meta-xylene recovery from a C8-aromatics stream.
Adsorptive separation units processing C8 aromatics generally use a simulated countercurrent movement of the adsorbent and the feed stream. This simulation is performed using established commercial technology wherein the adsorbent is held in place in one or more cylindrical adsorbent chambers and the positions at which the streams involved in the process enter and leave the chambers are slowly shifted along the length of the beds. A typical adsorptive separation unit is illustrated in FIG. 7 and includes at least four streams (feed, desorbent, extract and raffinate) employed in this procedure and the location at which the feed and desorbent streams enter the chamber and the extract and raffinate streams leave the chamber are simultaneously shifted in the same direction at set intervals. Each shift in location of the transfer points delivers or removes liquid to or from a different bed within the chamber. In general, to simulate countercurrent movement of the adsorbent relative to the fluid stream within the chamber, the streams are shifted in the general direction of fluid flow, i.e. the downstream direction, within the chamber to simulate the solid adsorbent moving in the opposite, i.e. upstream, direction. The lines at these transfer points are reused as each stream enters or leaves the associated bed, and each line therefore carries one of the four process streams during some point of the cycle.
The art recognizes that the presence of residual compounds in the transfer lines can have detrimental effects on a simulated-moving-bed process. U.S. Pat. Nos. 3,201,491; 5,750,820; 5,884,777; 6,004,518; and 6,149,874 teach the flushing of the line used to deliver the feed stream to the adsorbent chamber as a means to increase the purity of the recovered extract or sorbate component. Such flushing avoids contamination of the extract stream with raffinate components of the feed remaining in this line when it is subsequently used to withdraw the extract stream from the chamber. U.S. Pat. No. 5,912,395 teaches flushing of the line just used to remove the raffinate stream in order to avoid contaminating feed with raffinate when this line is used to deliver the feed stream to the adsorbent chamber. All of these references teach flushing such lines back into the adsorbent chamber, thus increasing the separation load within the chamber. U.S. Pat. No. 7,208,651 discloses flushing away from the adsorbent chamber the contents of a transfer line which previously has been used to remove the raffinate stream with one or both of a feed mixture and a material withdrawn from the adsorption zone. The residual raffinate within the transfer line is flushed to join the raffinate stream as feed to a raffinate column. U.S. Pat. No. 6,149,874 discloses flushing residual feed from a common section of fluid distribution piping to a booster circuit.
One previous exemplary system s utilized up to three flushes to handle residual fluid remaining in the transfer lines. A primary flush displaced residual extract from the transfer line just used to remove the extract stream with fluid from the desorption zone of the chamber just below the desorbent stream and directed it through a rotary valve to a transfer line just used to inject the feed stream. Because the volumes in the transfer lines were about equal, the extract-plus-desorbent fluid displaced the residual feed that had previously been in the transfer line into the adsorbent chamber just above the current feed stream position so that the residual feed could be separated with the feed stream within the adsorptive separation chamber and to avoid contamination of the extract stream with the residual feed remaining in the transfer line when the extract stream subsequently shifted to the transfer line previously occupied by the feed stream. Further, the residual extract from the primary flush used to displace the feed remained in the transfer line to be subsequently withdrawn by the extract stream to increase yield of the extract product.
The exemplary system sometimes included a secondary flush. The secondary flush utilized a flush of fluid, typically desorbent, through the transfer line and into the chamber immediately below the extract line. The secondary flush provided a “wash” of this transfer line with the desorbent to minimize the amount of contaminates, including raffinate, feed, and other components that may remain in the transfer line after the primary flush so that these materials were not withdrawn from the transfer line with the extract. Because this transfer line was previously flushed with desorbent and extract via the primary flush, the secondary flush was typically used in applications requiring high purity extract. The secondary flush would push the extract and desorbent material previously in the transfer line back into the adsorptive separation chamber. The secondary flush is an optional flush utilized to meet high purity demands of the extract product.
In some systems, a tertiary flush was also utilized. The tertiary flush included a flush of the transfer line previously occupied by the raffinate withdrawal stream. The tertiary flush was utilized to remove the residual raffinate from this transfer line to restrict this raffinate from being injected back into the adsorbent chamber with the feed upon subsequent arrival of the feed stream to the transfer line. Because the raffinate stream is depleted of the desired extract component, the tertiary flush was carried out so that the residual raffinate was not injected back into the adsorptive separation chamber, which would otherwise increase the separation demands in order to remove this additional raffinate material. The tertiary flush was accomplished by flushing the transfer line away from the adsorptive separation chamber with fluid from a port of the chamber adjacent to the transfer line.